Process for the fabrication of an inertial sensor with failure threshold

ABSTRACT

A process for the fabrication of an inertial sensor with failure threshold includes the step of forming, on top of a substrate of a semiconductor wafer, a sample element embedded in a sacrificial region, the sample element configured to break under a preselected strain. The process further includes forming, on top of the sacrificial region, a body connected to the sample element and etching the sacrificial region so as to free the body and the sample element. The process may also include forming, on the substrate, additional sample elements connected to the body.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for the fabrication ofan inertial sensor with failure threshold.

[0003] 2. Description of the Related Art

[0004] As is known, modern techniques of micromachining ofsemiconductors can be advantageously exploited for making variousextremely sensitive and precise sensors, having further small overalldimensions. The so-called MEMS sensors (ormicro-electro-mechanical-system sensors), are sensors that can beintegrated in a semiconductor chip and are suitable for detectingvarious quantities. In particular, both linear and rotational MEMSaccelerometers with capacitive unbalancing are known. In brief, theseaccelerometers are normally provided with a fixed body and of a mobilemass, both of which are conductive and are capacitively coupledtogether. In addition, the capacitance present between the fixed bodyand the mobile mass may vary, and its value depends upon the relativeposition of the mobile mass with respect to the fixed body. When theaccelerometer is subjected to a stress, the mobile mass is displacedwith respect to the fixed body and causes a variation in the couplingcapacitance, which is detected by a special sensing circuit.

[0005] As mentioned previously, MEMS accelerometers are extremelysensitive and precise; however, they are not suitable for being used inmany applications, mainly because they are complex to make and theircost is very high. On the one hand, in fact, the processes offabrication involve the execution of numerous non-standard steps and/orthe use of non-standard substrates (for example, SOI substrates); on theother hand, it is normally necessary to provide feedback sensingcircuits based upon differential charge amplifiers, the design of whichfrequently involves some difficulties.

[0006] In addition, in many cases the precision of capacitive MEMSsensors is not required and, indeed, it is not even necessary to have aninstantaneous measurement of the value of acceleration. On the contrary,it is frequently just necessary to verify whether a device incorporatingthe accelerometer has undergone accelerations higher than a pre-setthreshold, normally on account of impact. For example, the majority ofelectronic devices commonly used, such as cell phones, are protected bya warranty, which, however, is no longer valid if any malfunctioning isdue not to defects of fabrication but to an impact consequent on thedevice being dropped onto an unyielding surface or in any case on a usethat is not in conformance with the instructions. Unless visible damageis found, such as marks on the casing or breaking of some parts, it ispractically impossible to demonstrate that the device has suffereddamage that invalidates the warranty. On the other hand, portabledevices, such as cell phones, exactly, are particularly exposed to beingdropped and consequently to getting broken, precisely on account of howthey are used.

[0007] Events of the above type could be easily detected by an inertialsensor, which is able to record accelerations higher than a pre-setthreshold. However, the use of MEMS accelerometers of a capacitive typein these cases would evidently lead to excessive costs. It would thus bedesirable to have available sensors that can be made using techniques ofmicromachining of semiconductors, consequently having overall dimensionscomparable to those of capacitive MEMS sensors, but simpler as regardsboth the structure of the sensor and the sensing circuit. In addition,also the processes of fabrication should be, as a whole, simple andinexpensive.

BRIEF SUMMARY OF THE INVENTION

[0008] The purpose of the present invention is to provide a process forthe fabrication of an inertial sensor with failure threshold, which willenable the problems described above to be overcome.

[0009] According to an embodiment of the present invention, a process isprovided for the fabrication of an inertial sensor with failurethreshold, including the step of forming at least one sample elementembedded in a sacrificial region on top of a substrate of asemiconductor wafer, the sample element being configured to fractureunder a preselected force. The process further includes forming, on topof the sacrificial region, a body connected to the sample element, andetching the sacrificial region, so as to free the body and the sampleelement.

[0010] The process may include the step of making a weakened region ofthe sample element. The weakened region may be made by forming anarrowed region or notches in the sample element.

[0011] The process may include forming a plurality of sample elements,each configured to fracture under the preselected force.

[0012] According to an alternative embodiment of the invention, a methodfor manufacturing an inertial sensor is provided, comprising forming, ona semiconductor substrate, a sample element configured to break under apreselected strain, the sample element having a first end coupled to thesubstrate, and forming, above the semiconductor substrate, asemiconductor material body coupled to a second end of the sampleelement. The method may include forming a weakened region on the sampleelement, with the sample element configured to break at the weakenedregion under the preselected strain.

[0013] According to this embodiment, the sample element may have a Tshape, the first end being the cross-bar portion of the T and beingcoupled to the substrate at extreme ends thereof, the second end beingthe upright portion of the T.

[0014] The method may also include forming an additional sample elementhaving a first end coupled to the substrate, a second end coupled to thesemiconductor material body, and configured to break under thepreselected strain.

[0015] According to another embodiment of the invention, A method ofmeasuring movement of a device is provided, including providing acircuit in the device configured to permanently change the conductivestate of a conductive path in the event the device is subjected to anacceleration exceeding a preselected level, applying a potential atfirst and second ends of the conductive path, and detecting a change inthe conductive state of the conductive path.

[0016] The method may further include breaking a semiconductor structurethrough which the conductive path passes in the event the device issubjected to the acceleration. This step may be performed by moving afirst semiconductor body relative to a second semiconductor body inresponse to inertial forces resulting from the acceleration, thesemiconductor structure being coupled at a first end thereof to thefirst body and at a second end to the second body, the movement of thefirst body causing a flexion of the structure, resulting in the breakingthereof.

[0017] The device may be a cell phone, and the preselected level may beselected to correspond to an acceleration caused by a drop of the deviceto an unyielding surface from a preselected height. The preselectedlevel may also be selected to be equal to or less than an accelerationsufficient to damage the device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0018] For a better understanding of the invention, some embodimentsthereof are now described, purely by way of non-limiting examples andwith reference to the attached drawings, in which:

[0019]FIGS. 1 and 2 are cross-sectional views through a semiconductorwafer in successive steps of fabrication in a first embodiment of theprocess according to the present invention;

[0020]FIG. 3 is a top plan view of the wafer of FIG. 2;

[0021]FIG. 4 illustrates an enlarged detail of FIG. 3;

[0022]FIG. 5 is a cross-sectional view of the wafer of FIG. 3 in asubsequent fabrication step;

[0023]FIG. 6 is a top plan view of the wafer of FIG. 5;

[0024]FIGS. 7 and 8 are cross-sectional views of the wafer of FIG. 6 ina subsequent fabrication step, taken along the planes of trace VII-VIIand VIII-VIII, respectively, of FIG. 6;

[0025]FIG. 9 is a top plan view of the wafer of FIG. 7, in a subsequentfabrication step, in which an inertial sensor is obtained;

[0026]FIGS. 10 and 11 are cross-sectional views of the wafer of FIG. 9,taken along the planes of trace X-X and XI-XI, respectively, of FIG. 9;

[0027]FIGS. 12 and 13 are cross-sectional views through a compositewafer and a die, respectively, obtained starting from the wafer of FIG.9;

[0028]FIG. 14 is a schematic view of the top three quarters of a deviceincorporating the die of FIG. 13;

[0029]FIG. 15 is a schematic illustration of an inertial sensor of thetype illustrated in FIGS. 9-13 in an operative configuration;

[0030]FIG. 16 is a detail of an inertial sensor obtained according to avariant of the first embodiment of the present process;

[0031]FIG. 17 is a top plan view of an inertial sensor obtainedaccording to a further variant of the first embodiment of the presentprocess;

[0032]FIG. 18 is a cross-sectional view of the sensor of FIG. 17;

[0033]FIG. 19 is a top plan view of an inertial sensor obtainedaccording to a second embodiment of the present invention;

[0034]FIG. 20 illustrates an enlarged detail of FIG. 19;

[0035]FIG. 21 is a top plan view of an inertial sensor obtainedaccording to a third embodiment of the present invention;

[0036]FIG. 22 illustrates an enlarged detail of FIG. 21;

[0037]FIG. 23 is a schematic illustration of two inertial sensors of thetype illustrated in FIG. 21 in an operative configuration;

[0038]FIG. 24 is a cross-sectional view through a semiconductor wafer inan initial fabrication step of a process according to a fourthembodiment of the present invention;

[0039]FIG. 25 is a top plan view of the wafer of FIG. 24;

[0040]FIG. 26 illustrates the wafer of FIG. 24 in a subsequentfabrication step;

[0041]FIG. 27 is a top plan view of the wafer of FIG. 26 in a subsequentfabrication step, in which an inertial sensor is obtained;

[0042]FIG. 28 is a cross-sectional view through the wafer of FIG. 27,taken according to the plane of trace XXVI-XXVI of FIG. 27;

[0043]FIG. 29 is a plan view of a detail of an inertial sensor obtainedaccording to a fifth embodiment of the present invention

[0044]FIG. 30 is a side view of the detail of FIG. 29; and

[0045]FIG. 31 is a side view of the detail of FIG. 29, obtainedaccording to a variant of the fifth embodiment of the present invention.

DESCRIPTION OF THE INVENTION

[0046] With reference to FIGS. 1-13, a wafer 1 of semiconductormaterial, for example monocrystalline silicon, comprises a substrate 2,on which a thin pad oxide layer 3, for example 2.5 μm thick, isthermally grown. A conductive layer 5 of polysilicon, having for examplea thickness of between 400 and 800 nm and a dopant concentration of 10¹⁹atoms/cm³, is then deposited on the pad oxide layer 3 and is defined bymeans of a photolithographic process. Two T-shaped samples 6 are thusobtained, having respective feet 6 a, aligned with respect to oneanother and extending towards one another, and respective arms 6 bparallel to one another (FIGS. 2-4). The feet 6 a and the arms 6 b ofeach sample 6 are set in directions identified by a first axis X and,respectively, by a second axis Y, which are mutually orthogonal (a thirdaxis Z, orthogonal to the first axis X and the second axis Y, isillustrated in FIG. 2). In addition, at respective ends of the arms 6 bof both the samples 6 anchoring pads 8 are made, of a substantiallyrectangular shape and having a width greater than the arms 6 b. Asillustrated in FIG. 4, each of the samples 6 has a first weakened region9 and a second weakened region 10. In particular, in both of the samples6, the first weakened region 9 and the second weakened region 10 aremade as narrowed portions of the foot 6 a and, respectively, of one ofthe arms 6 b. In addition the weakened regions 9, 10 are defined bynotches 11 with a circular or polygonal profile, made in an area ofjoining 6 c between the foot 6 a and the arms 6 b and traversing thesample 6 in a direction parallel to the third axis Z. The thickness ofthe conductive layer 5 of polysilicon, the dimensions of the feet 6 aand of the arms 6 b of the samples 6, and the conformation of theweakened regions 9, 10 determine the mechanical resistance to failure ofthe samples 6 themselves. In particular, acting on the shape and on thedimensions of the notches 11 defining the first weakened region 9 andthe second weakened region 10, it is possible to obtain pre-set failurethresholds of the samples 6 along the first, second and third axes X, Yand Z. Preferably, all the mechanical failure thresholds are basicallythe same.

[0047] Next, a sacrificial layer 12 of silicon dioxide is deposited soas to coat the pad oxide layer 3 and the samples 6. In practice, the padoxide layer 3 and the sacrificial layer 12 form a single sacrificialregion in which the samples 6 are embedded. The sacrificial layer 12 isthen defined by means of a photolithographic process comprising twomasking steps. During a first step, first openings 14 are made in thesacrificial layer 12, exposing respective ends of the feet 6 a of thesamples 6, as illustrated in FIG. 5. In a second step of thephotolithographic process (FIG. 6), both the sacrificial layer 12 andthe pad oxide layer 3 are selectively etched, so as to make secondopenings 15, exposing portions of the substrate 2.

[0048] Subsequently, a conductive epitaxial layer 16 is grown on thewafer 1, the said layer having a thickness, for example, of 15 μm and adopant concentration of 10¹⁸ atoms/cm³. In detail, the epitaxial layer16 coats the sacrificial layer 12 entirely and extends in depth throughthe first and the second openings 14, 15 until the samples 6 and thesubstrate 2, respectively, are reached (FIG. 7 and 8).

[0049] The epitaxial layer 16 is then selectively etched, preferably byreactive-ion etching (RIE), and the sacrificial layer 12 and the padoxide layer 3 are removed. In greater detail, during the step of etchingof the epitaxial layer 16, the following are formed: a mobile mass 18;anchorages 19, provided on the portions of the substrate 2 previouslyexposed by the second openings 15; a plurality of springs 20, connectingthe mobile mass 18 to the anchorages 19; and a ring-shaped supportingstructure 21, which surrounds the mobile mass 18, the samples 6, thesprings 20, and the corresponding anchorages 19 (see FIG. 9, in whichthe sacrificial layer 12 and the pad oxide layer 3 have already beenremoved).

[0050] The mobile mass 18 is connected to the substrate 2 by the springs20, which are in turn constrained to the anchorages 19 (FIG. 11). Thesprings 20, which are per se known, are shaped so as to enableoscillations of the mobile mass 18 with respect to the substrate 2 alongeach of the three axes X, Y, Z, at the same time, however, preventingrotations. The mobile mass 18 is moreover constrained to the substrate 2through the samples 6. In greater detail, the mobile mass 18 has, in amedian portion, a pair of anchoring blocks 22, projecting outwards inopposite directions along the second axis Y. The anchoring blocks 22 areconnected to the end of the foot 6 a of a respective one of the samples6, as illustrated in FIG. 10. In turn, the samples 6 are anchored to thesubstrate 2 through the anchoring pads 8. By controlling the duration ofetching of the sacrificial layer 12 and of the pad oxide layer 3, thesilicon dioxide is in fact removed only partially underneath theanchoring pads 8, which are wider than the feet 6 a and the arms 6 b ofthe samples 6; thus, residual portions 3′ of the pad oxide layer 3,which are not etched, fix the anchoring pads 8 to the substrate 2,serving as bonding elements.

[0051] The sacrificial layer 12 and the remaining portions of the padoxide layer 3 are, instead, completely removed and, hence, the mobilemass 18 and the samples 6 are freed. In practice, the mobile mass 18 issuspended at a distance on the substrate 2 and can oscillate about aresting position, in accordance with the degrees of freedom allowed bythe springs 20 (in particular, it can translate along the axes X, Y andZ). Also the samples 6 are elastic elements, which connect the mobilemass 18 to the substrate 2 in a way similar to the springs 20. Inparticular, the samples are shaped so as to be subjected to a stresswhen the mobile mass 18 is outside a relative resting position withrespect to the substrate 2. The samples 6 are, however, very thin andhave preferential failure points in areas corresponding to the weakenedregions 9, 10. For this reason, their mechanical resistance to failureis much lower than that of the springs 20, and they undergo failure in acontrolled way when they are subjected to a stress of pre-set intensity.

[0052] In practice, at this stage of the process, the mobile mass 18,the substrate 2, the springs 20 with the anchorages 19, and the samples6 form an inertial sensor 24, the operation of which will be describedin detail hereinafter.

[0053] An encapsulation structure 25 for the inertial sensor 24 is thenapplied on top of the wafer 1, forming a composite wafer 26 (FIG. 12).In particular, the encapsulation structure 25 is an additionalsemiconductor wafer, in which a recess 27 has previously been opened, ina region that is to be laid on top of the mobile mass 18. Theencapsulation structure 25 is coupled to the ring-shaped supportingstructure 21 by the interposition of a layer of soldering 29. Next, thecompound wafer 26 is cut into a plurality of dice 30, each diecomprising an inertial sensor 24 and a respective protective cap 31,formed by the fractioning of the encapsulation structure 25 (FIG. 13).

[0054] The die 30 is finally mounted on a device 32, for example a cellphone. Preferably, the device 32 is provided with a casing 33, insidewhich the die 30 is fixed, as illustrated in FIG. 14. In addition (FIG.15), the inertial sensor 24 is connected to terminals of a testingcircuit 35, which measures the value of electrical resistance betweensaid terminals. In greater detail, the anchoring pads 8 of the arms 6 b,in which the second weakened regions 10 are formed, are connected eachto a respective terminal of the testing circuit 35.

[0055] In normal conditions, i.e., when the inertial sensor 24 isintact, the samples 6 and the mobile mass 18 form a conductive path thatenables passage of current between any given pair of anchoring pads 8.In practice, the testing circuit 35 detects low values of electricalresistance between the anchoring pads 8. During normal use, the device32 undergoes modest stresses, which cause slight oscillations of themobile mass 18 about the resting position, without jeopardizing theintegrity of the inertial sensor 24.

[0056] When the device 32 suffers a shock, the mobile mass 18 of theinertial sensor 24 undergoes a sharp acceleration and subjects thesamples 6 and the springs 20 to a force. According to the intensity ofthe stress transmitted to the inertial sensor 24, said force can exceedone of the thresholds of mechanical failure of the samples 6, whichconsequently break. In particular, failure occurs at one of the weakenedregions 9, 10, which have minimum strength. In either case, theconductive path between the two anchoring pads 8 connected to thetesting circuit 35 is interrupted, and hence the testing circuit detectsa high value of electrical resistance between its own terminals, thusenabling recognition of the occurrence of events that are liable todamage the device 32.

[0057] According to a variant of the embodiment described, shown in FIG.16, T-shaped samples 37 are provided, which present a single weakenedregion 38. In particular, the weakened region 38 is a narrowed portiondefined by a pair of notches 39, which are oblique with respect to afoot 37 a and arms 37 b of the samples 37.

[0058] According to a further variant, illustrated in FIGS. 17 and 18,the two T-shaped samples 6 are located in a gap 36 between the substrate2 and the mobile mass 18 and have the end of the respective feet 6 a inmutual contact. In addition, both of the samples 6 are fixed to a singleanchoring block 22′ set centrally with respect to the mobile mass 18itself.

[0059] The process according to the invention has the followingadvantages. In the first place, for fabrication of the inertial sensor24, processing steps that are standard in the microelectronics industryare employed. In particular, the following steps are carried out: stepsof deposition of both insulating and conductive layers of material;photolithographic processes; a step of epitaxial growth; and standardsteps of etching of the epitaxial silicon and of the insulating layers.Advantageously, a single step of thermal oxidation is carried out, andconsequently the wafer 1 is subjected to modest stresses during thefabrication process. The yield of the process is therefore high. Inaddition, the inertial sensor 24 is obtained starting from a standard,low-cost substrate.

[0060] The process described consequently enables inertial sensors withfailure threshold to be produced at a very low cost. Such sensors areparticularly suitable for use where it is necessary to record theoccurrence of stresses that are harmful for a device in which they areincorporated and in which it is superfluous to provide precisemeasurements of accelerations. For example, they can be advantageouslyused for verifying the validity of the warranty in the case of widelyused electronic devices, such as, for example, cell phones.

[0061] In addition, the inertial sensors provided with the presentmethod have contained overall dimensions. In inertial sensors, in fact,large dimensions are generally due to the mobile mass, which must ensurethe necessary precision and sensitivity. In this case, instead, it issufficient that, in the event of a predetermined acceleration, themobile mass will cause breaking of the weakened regions of the samples,which have low strength. It is consequently evident that also the mobilemass can have contained overall dimensions.

[0062] The use of a single anchoring point between the samples and themobile mass, as illustrated in the second variant of FIGS. 17 and 18,has a further advantage as compared to the ones already pointed out,because it enables more effective relaxation of the stresses due toexpansion of the materials. In particular, it may happen that thepolysilicon parts which are even only partially embedded in the silicondioxide (samples and portions of the epitaxial layer) will be subjectedto a compressive force, since both the polysilicon, and the oxide tendto a expand in opposite directions during the fabrication process. Whenthe oxide is removed, the action of compression on the polysilicon iseliminated, and the polysilicon can thus expand. Clearly, the largestexpansion, in absolute terms, is that of the mobile mass, since it hasthe largest size. The use of a single anchoring point, instead of twoanchorages set at a distance apart enables more effective relaxation ofthe stresses due to said expansion, since the mobile mass can expandfreely, without modifying the load state of the samples.

[0063] The inertial sensors obtained using the process described aremore advantageous because they respond in a substantially isotropic wayto the mechanical stress. In practice, therefore, just one inertialsensor is sufficient to detect forces acting in any direction.

[0064] A second embodiment of the invention is illustrated in FIGS. 19and 20, where parts that are the same as the ones already illustratedare designated by the same reference numbers. According to saidembodiment, an inertial sensor 40 is made, having L-shaped samples 41.As in the previous case, the samples 41 are obtained by shaping aconductive polysilicon layer deposited on top of a pad oxide layer (notillustrated herein), which has in turn been grown on the substrate 42 ofa semiconductor wafer 43. Using processing steps similar to the onesalready described, the mobile mass 18, the anchorages 19 and the springs20 are subsequently obtained.

[0065] In detail, the samples 41 have first ends connected to respectiveanchoring blocks 22 of the mobile mass 18, and second ends terminatingwith respective anchoring pads 41 fixed to the substrate 2, as explainedpreviously. In addition, notches 42 made at respective vertices 43 ofthe samples 41 define weakened regions 44 of the samples 40.

[0066]FIGS. 21 and 22 illustrate a third embodiment of the invention,according to which an inertial sensor 50 is obtained, made on asubstrate 54 and provided with substantially rectilinear samples 51 thatextend parallel to the first axis X. In this case, during the RIEetching step, in addition to the mobile mass 18, two anchorages 52 andtwo springs 53 of a known type are provided, which connect the mobilemass 18 to the anchorages 52 and are shaped so as to preventsubstantially the rotation of the mobile mass 18 itself about the firstaxis X.

[0067] The samples 51 have first ends soldered to respective anchoringblocks 22 of the mobile mass 18 and second ends terminating withanchoring pads 55, made as described previously. In addition, pairs oftransverse opposed notches 57 define respective weakened regions 58along the samples 51 (FIG. 22).

[0068] Alternatively, the weakened regions may be absent.

[0069] The inertial sensor 50 responds preferentially to stressesoriented according to a plane orthogonal to the samples 51, i.e., theplane defined by the second axis Y and by the third axis Z. In thiscase, to detect stresses in a substantially isotropic way, it ispossible to use two sensors 50 connected in series between the terminalsof a testing circuit 59 and rotated through 90° with respect to oneanother, as illustrated in FIG. 23.

[0070] With reference to FIGS. 24-28, according to a fourth embodimentof the invention, a pad oxide layer 62 is grown on a semiconductor wafer60 having a substrate 61. Next, a conductive layer 63 of polycrystallinesilicon (here indicated by a dashed line) is deposited on the pad oxidelayer 61 and is defined to form a sample 64, which is substantiallyrectilinear and extends parallel to the first axis X (FIG. 25). Thesample 64 has an anchoring pad 65 at one of its ends and has a weakenedregion 66 defined by a pair of notches 67 in a central position.

[0071] A sacrificial layer 69 of silicon dioxide is deposited so as tocoat the entire wafer 60 and is then selectively removed to form anopening 68 at one end of the sample 64 opposite to the anchoring pad 65.

[0072] An epitaxial layer 70 is then grown (FIG. 26), which is etched soas to form a mobile mass 71, anchorages 72, springs 73, and a supportingring (not illustrated for reasons of convenience). The epitaxial layerextends into the opening 68 to form a connection region 88 between thesample 64 and the mobile mass 71.The sacrificial layer 69 and the padoxide layer 62 are removed, except for a residual portion 62′ of the padoxide layer 62 underlying the anchoring pad 65 (FIGS. 27 and 28). Themobile mass 71 and the sample 64 are thus freed. More precisely, themobile mass 71, which has, at its center, a through opening 74 on top ofthe sample 64, is constrained to the substrate 61 through the anchorages72 and the springs 73, which are shaped so as to prevent any translationalong or rotation about the first axis X. In addition, the sample 65 hasopposite ends, one connected to the substrate 2 through the anchoringpad 65, and the other to the mobile mass 71 at connection region 88, andis placed in a gap 76 comprised between the mobile mass 71 and thesubstrate 61.

[0073] In this way, an inertial sensor 80 is obtained, which is thenencapsulated through steps similar to the ones described with referenceto FIGS. 12 and 13.

[0074] Also in this case, the use of a single anchoring point betweenthe sample and the mobile mass advantageously enables effectiverelaxation of the stresses due to expansion of the mobile mass.

[0075] According to one variant (not illustrated), the sample isT-shaped, like the ones illustrated in FIG. 9.

[0076]FIG. 29 illustrates a detail of a sample 81, for example arectilinear one, of an inertial sensor obtained using a fifth embodimentof the process according to the invention. In particular, the sample 81has a weakened region defined by a transverse groove 82 extendingbetween opposite sides 83 of the sample 81.

[0077] The groove 82 is obtained by means of masked etching ofcontrolled duration of the sample 81 (FIG. 30).

[0078] Alternatively (FIG. 31), a first layer 85 of polysilicon isdeposited and defined. Then, a stop layer 86 of silicon dioxide and asecond layer 87 of polysilicon are formed. Finally, a groove 82′ is dugby etching the second layer 87 of polysilicon as far as the stop layer86.

[0079] Finally, it is evident that modifications and variations may bemade to the process described herein, without thereby departing from thescope of the present invention. In particular, the weakened regions canbe defined by using side notches in the samples together with groovesextending between the side notches. In addition, the weakened regionscould be defined by through openings that traverse the samples, insteadof by side notches.

[0080] All of the above U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet, areincorporated herein by reference, in their entirety.

[0081] From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A process for the fabrication of an inertial sensor with failurethreshold, comprising the steps of: forming, on top of a substrate of asemiconductor wafer, at least one sample element embedded in asacrificial region; forming, on top of said sacrificial region, a bodyconnected to said sample element; and etching said sacrificial region,so as to free said body and said sample element.
 2. The processaccording to claim 1, in which the step of forming said sample elementcomprises: forming a first layer of a first material, which coats saidsubstrate; forming a second layer of a second material, which coats saidfirst layer; shaping said second layer, so as to define said sampleelement; and forming a third layer of said first material coating saidfirst layer and said sample element.
 3. The process according to claim2, in which said first material is a dielectric material and said secondmaterial is a conductive material.
 4. The process according to claim 3,in which said first material is silicon dioxide and said second materialis polysilicon.
 5. The process according to claim 1 wherein the step offorming at least one sample element comprises the step of making atleast one weakened region of said sample element.
 6. The processaccording to claim 5, in which the step of making at least one weakenedregion comprises the step of defining a narrowing of said sampleelement.
 7. The process according to claim 6 in which said step ofdefining a narrowing portion comprises forming notches in said sampleelement.
 8. The process according to claim 5 in which the step of makingat least one weakened region comprises making a groove extending betweenopposite edges of said sample element.
 9. The process according to claim8, in which the step of making a groove comprises performing an etch ofcontrolled duration of said sample element.
 10. The process according toclaim 8 in which the step of making a groove comprises: forming a stoplayer inside said sample element; and etching said sample element untilsaid stop element is reached.
 11. The process according to claim 1wherein the step of forming at least one sample element comprisesdefining at least one anchoring pad of said sample element.
 12. Theprocess according to claim 11, in which the step of etching saidsacrificial region is interrupted before removing residual portions ofsaid sacrificial region underlying said anchoring pad.
 13. The processaccording to claim 1, further comprising making, before performing thestep of forming said body, at least one first opening through saidsacrificial region, which exposes one end of said sample element, andmaking second openings, which expose respective portions of saidsubstrate.
 14. The process according to claim 13, in which the step offorming said body comprises: growing an epitaxial layer, which extendson top of said sacrificial region and through said first opening andsaid second openings; and etching said epitaxial layer until saidsacrificial region is reached.
 15. The process according to claim 14, inwhich, during the step of etching said epitaxial layer there are definedanchorages connected to said substrate and elastic elements connectingsaid body to said anchorages.
 16. A method for manufacturing an inertialsensor, comprising: forming, on a semiconductor substrate, a sampleelement having a first end coupled to the substrate, the sample elementbeing configured to break under a preselected strain; and forming, abovethe semiconductor substrate, a semiconductor material body coupled to asecond end of the sample element.
 17. The method of claim 16 wherein thesample element has a T shape, the first end forming a cross-bar portionof the T and being coupled to the substrate at extreme ends of thecrossbar, the second end extending from a central portion of thecrossbar to form the T.
 18. The method of claim 16, further comprisingforming an additional sample element having a first end coupled to thesubstrate, a second end coupled to the semiconductor material body, andconfigured to break under the preselected strain.
 19. The method ofclaim 16, further comprising forming a weakened region on the sampleelement, and wherein the sample element is configured to break at theweakened region under the preselected strain.
 20. The method of claim 19wherein the weakened region comprises a narrowed region of the sampleelement.
 21. A method of measuring movement of a device, comprising:providing, in the device, a circuit configured to permanently change aconductive state of a conductive path in the event the device issubjected to an acceleration exceeding a preselected level; applying apotential at first and second ends of the conductive path; and detectinga change in the conductive state of the conductive path.
 22. The methodof claim 21 wherein the circuit is configured to break the conductivepath.
 23. The method of claim 21 wherein the device is a cellular phone.24. The method of claim 21 wherein the preselected level corresponds toan acceleration caused by a drop of the device to an unyielding surfacefrom a preselected height.
 25. The method of claim 21 wherein thepreselected level is selected to be equal to or less than anacceleration sufficient to damage the device.
 26. The method of claim21, further comprising breaking a semiconductor structure through whichthe conductive path passes in the event the device is subjected to theacceleration.
 27. The method of claim 26 wherein the breaking stepcomprises moving a first semiconductor body relative to a secondsemiconductor body in response to inertial forces resulting from theacceleration, the semiconductor structure being coupled at a first endthereof to the first body and at a second end to the second body, themovement of the first body causing a flexion of the structure, resultingin the breaking thereof.
 28. The method of claim 27 wherein the secondsemiconductor body is rigidly coupled to the device.